I've been interested in boats using the Wing In Ground Effect for a
couple decades. The low-energy soaring of pelicans near waves, the
ground effect on a frisbee floating instead of landing after gliding
down near to the ground, and my ground-school teacher explaining
ground effect to cousin Rosanna and me in 2001, have inspired me to
think about this subject. The Russians call it the "screen", or
"ekran", as though there is a kind of screen along the surface of the
ground or water, that lets you through and partly doesn't let you
through, as you're landing.|
Even before that, my friend Norbert Wu's father, who was an
aerodynamics professor at Georgia Tech, had explained to me once about
vortex generation under bird wings, and how commercial aviation is
trying to figure out how to use that tremendous lifting power.
Driving home from ground school with Rosanna, I started thinking of
designs of a boat with wings.
My first thoughts had water propellers, and a canard wing in
It seemed to me that a small wing out front, a canard, could generate
a downdraft below the main wing, producing greater pressure there and
Fig. 10, photo of the Odessa Institute of Merchant Marine Engineers'
WIG boat under Yu. A. Budnitsky.)
And I thought, a water prop might be more efficient because it's
pushing against an incompressible and dense material, water, which
ought to be better for mechanical energy transfer than pushing against
a compressible light material, air. (N. I. Belavin
14 reports that about double the thrust comes from water props as
compared with air.)
Also WIG boats have a limited safe
velocity range, so it might be an advantage if the power mechanism
disengages when the boat flies up too high, and the prop comes out of
the water. But apparently air props can be efficient too. Finally,
the efficiency of a high-speed boat prop depends on the blade being
half in and half out of the water, a very very precise elevation
requirement, which even medium waves will prevent. So I'm open to air
Sponsons in front
Curran's Lehigh University thesis on aerodynamics of high-speed
sponsons influenced my thinking: the front left and right
water-contacting corners of a WIG boat should be a bit like Curran's
Sponson A, that is, aerodynamically neutral in varying angles of
attack, but hydrodynamically efficient, with a step to reduce wetted
area at speed.
Three points of contact
A balanced WIG boat should work well as a high-speed boat also, with
stability and efficiency operating in full water contact. For high
speed, I like wide-set front sponsons and rear drive. (Check
example, I'll call it the 2015 Edderitz boat.)
RCTestFlight: a model to emulate
I love this guy's work: ground effect vehicle over snow and
vehicle. Can't find his actual name to give credit but his
YouTube ID is RCTESTFLIGHT. You might say his experiments are
primitive but I say they are brilliant and moving things in the right
I like it that his prop is above the wing because higher speed air is
lower pressure so more lift up there. These designers with PAR (power
assisted reinjection of air under the wing) are creating the wrong
- I believe the excellent lift and easy-liftoff qualities of this
vehicle are due to the Coanda effect whereby the outflow of the
propeller above the wing entrains onto the upper surface of the
wing, hence, due to the Bernoulli effect, provides extreme lift
to the wing in addition to forward propulsion.
- You can see the propeller axis points somewhat downward toward
the top of the wing surface, thus making it easier for the
Coanda entrainment to occur.
- This vehicle's wing has an extremely long chord. Why?
In Coanda/Bernoulli lift enhancement the propeller outwash
entrains onto the wing upper surface. The total lift force is
proportional to the amount of wing surface to which the
resulting lowered pressure applies. So you could increase lift
by increasing the upper surface area impacted by the outwash,
that is essentially by either increasing width (span) or length
(chord) of the wing.
For more, read
flaps, especially look for the phrase Upper Surface Blow,
which in the one built case, the YC-14, increased the
coefficient of lift to 7, (compare with a Boeing 747 at high
altitude cruise having a coefficient of lift of 0.52 according
- Increase the wing span, and nothing happens, because the
outwash is centered on the propeller and can only spread
laterally a limited amount. You could increase the amount
of the span which is affected by using multiple
propellers, or by providing ducts or vanes to spread the
propulsion wash laterally across the wing. Ugh. Or
- Increase the wing chord (increasing the duration of
airflow contact with the wing), and you directly increase
the area effected by Coanda/Bernoulli. Thus more lift.
The effect occurs over a larger area with a longer
above-wing wash entrainment surface. It lowers the
pressure above the wing over a larger total surface area.
- So the outwash is essentially a narrow (though widening)
but long resource for imposing lift onto a wing. Actually
its shape is likely somewhat triangular, with greater span
influenced as the wash reaches along a greater distance of
To fix the one-wing-in-contact-with-the-water issue, I say, turn
the pontoon into a knife edge so it has minimum drag and go ahead and
let it contact the water in normal operation
His snow-sled flies nicely in ground effect with
estimated dimensions 1: rear vane height and length; 3 sled length;
2.5 sled width; 0.8: propeller axis elevation above sled frame; 2:
distance from sled tail to delta wing front root; .75: distance from
sled tail to delta wing front outside corner. 1.25 width of delta
wing, each side. Rear pitch-control surface: full width, 0.5
length. 2.5/12 slope of propeller axis. He has end pontoons, full
Materials to build experimental systems
- For hand-made RC prototypes: Blue or pink insulation foam. EPP foam. 6mm Devron foam. 1/2" wood dowels (square).
Grayson Hobby Super mega Jet electric motor.80Amp ESC. Nanotech 3S lipopack 2200. or 1300mAh 3 cell battery. Glued receiver for waterproofing. Servos to turn control surfaces (rudder). Blue Wonder motors. Radio controller: DSM2 DX7 Model Match[TM] technology. 3S brushless motor with watercooling for high speed boats.
- For full size systems: aluminum boat building technology.
What to call them
Naming is a struggle for this category of most
fascinating vehicles. It has the worst names: ekranoplan or
screen-plane in Russian, WIG, AGEC, WISES, these names suck.
I don't have a solution, though I don't mind airboat or
wingboat. Anyhow W-In-G sounds more like WING than WIG to me.
Whatever, just use all the names in your web pages so everyone
can find it, whatever you want to call it. I hate it that
we're stuck with WIG Boat.
Death by Center-of-lift variation.
Ok here are some ways to hopefully not die.
If you are just designing a fast boat:
In our case (WIG boats), the basic thing is that if your ground
effect is from air stagnation under the wing, then the lift
comes primarily at the trailing edge of the wing where all that
air is finally maximally crushed under the trailing edge, and
that pushes the vehicle up. The air at the front part isn't
compressed yet, so it doesn't push upward that much. So
stagnation effect lift is located emphatically rearward. Then
as soon as the vehicle rises out of stagnation ground effect,
the air isn't nearly as compressed at the trailing edge, and
the whole wing contributes more equally to the total effect,
irrespective of front or back, so the center of lift moves
forward. Therefore once the vehicle catches some lift to get
1/4 or more of a wingspan up, or especially if in the back it
gets up even just a little, then the relative compression at
the trailing edge drops off dramatically. Compare the thin edge
of air under a 5-percent elevation-to-span ratio scenario, for
example. When the center of lift suddenly distributes forward
to the center of the wing, because you were balanced on the
rear-edge center of lift, now suddenly the whole thing flips up
and backwards and you
Nice way to die, is what I'm saying. This is a problem.
- make sponsons in front (or
= face-plant crash with sponsons not in front) and
- make them
aerodynamically neutral (or
- backflip crash with sponsons in front but not aerodynamically
So what's the solution? Lippisch by his designs said, spread
out the rear edge in a front-back dimension by making the rear
edge sharply diagonal forward, so that a good part of the
"rear" edge is quite a bit forward of the rearmost part of the
rear edge. He also added a small pontoon wing also up front,
to add more front-loaded lift to the mix. Then the change will
be less when the nose kicks up.
But consider de-emphasizing the stagnation (chord-dominated)
ground effect, and instead use span-dominated ground effect
lift (where ground effect efficiency comes from reduced wing
downwash in proximity to the ground surface) or plain lift or a
blown wing top in your design.
My thought is, use a blown wing top with prop above and long
chord along the centerline of the vehicle body -- and at the
same time a long span. Think Lippisch X-112 without the delta
shape, just a long chord for a middle section of wing, while
emphasizing extra long pontoon wings.
In idle imagination it comes to me as a biplane, with Coanda
lift on an upper, long-chord, short-span wing, perhaps containing
the passenger/cargo spaces within the wing itself, all above a
lower, long span wing using ground effect lift. When the prop
blows harder, the plane will lift hard and may rotate forward
like an accelerating helicopter. With reduced prop power,
ground effect on the lower wing still keeps you floating
- Comparing the two sources of ground effect, span-dominated versus
chord-dominated, the main effect is span-dominated. Van Opstal's
graph shows that at a height of 5% of wingspan, the drag is
reduced by to a 30% fraction compared with free flight. Whereas at a
height of 5% of the chord, the drag reduces only from 1.1 to
0.8 as compared with free flight. 5% of the CHORD! It would require a
chord longer than the average wingspan to get far less than half the
effect, if my reasoning is correct. Therefore ground effect is hugely
dominated by the wing span effect. Another way to say this is that
wing downwash is a bigger energy sink than wingtip vortices. And
therefore making a long-chord wing, which many designs are based on
including Lippisch and Jorg, is a mistake; whereas a short-chord,
longer span wing, is the way to maximize ground effect.
Witness low-soaring birds such as seagulls and pelicans: Long narrow
If the Alexeev ekranoplan designs have short wings, it's because the
lift is so great they don't need more.
Water Contact and Take Off Power
- Water contact can be more than a drag. If wave height is
normally distributed, then occasional freak waves are inevitable,
and the design of WIG boats should allow for non-catastrophic
water contact during normal flight. This means knifing sponsons
in the front corners where the bounce off the water is pretty
- The much-higher power requirement for take-off might be remedied
by externally-applied acceleration: a slingshot launch method.
Imagine the WIG-boat can putt-putt along the water, or fly above
it, but can't make the transition without help. That's safe but
if not necessarily convenient, because if you fall out of the sky
you can still putt-putt along to wherever you need to
- I started with a water prop concept when I first started
imagining these. Google the Kawasaki KAG-3
- Water contact is a drag. Well, based on water density = 1728x
steam density, I think a vehicle that transitions smoothly
between water phase and air phase lift, propulsion, and vaning,
should have roughly 1000x larger air surfaces.
- Imagine two motors, one for an air prop and one for a water
prop; the air prop pushes pi*Ra*Ra*Da in one rotation, which
should be 1000x more volume than the water prop's
pi*Rw*Rw*Dw. So if both screw forward through the same
height of cylinder of the medium in one propeller rotation,
then Dw = Da. Equating and cancelling, we have Rw*Rw*1000 =
Ra*Ra or Rw*32=Ra. Thus if a hovercraft pusher prop has
radius 32 in. (diameter 64 in.), then the equivalent water prop
should have radius 1 in. (diameter 2 in.). I think I must be
missing some factor, perhaps the Dw=Da assumption is wrong,
or the normal RPMs are lower in the water, because this
result seems a little too far in the direction of my
argument that only a small water prop is required to get
plenty of propulsion, compared with an air prop.
- Similarly an air wing should pass over and push downward a
volume of 1000x the water that a hydrofoil should effect.
If the same square area relationship applies, then an air
wing of say 3.2'x10' would be the equivalent of a hydrofoil
lifting surface of 6-3/4 in. x 21-1/4 in.. I don't trust my
reasoning here at all. But it tends in the right direction,
that a very small lifting surface in the water would be
equivalent to a reasonably large air wing.
- Finally, considering vaning effects as by a rudder (in WIGs
we should pretty automatically have attitude stability or we
have a bad design), here a tiny rudder in the water has the
effect of an enormous rudder in the air. To get the 32
square feet of surface for an air rudder shaped as a crude
approximation like an isosceles right triangle, we need 8'
legs on the triangle! (8 x 8 = 64 = area of a square, the
isosceles right triangle is half that square).
- So just consider a water prop for in-the-water putt-putt
movement. Double the thrust for the same horsepower, sure why
not. You might need a separate motor, or some efficient means of
transferring work from a front air prop to a rear water propeller
- But a blown wingtop seems promising. Make the prop wash entrain
to the top of the wing via the Coanda effect. Then it should
will produce huge lift, according to the Bernoulli effect. 14x
maybe. Shouldn't that be of some assistance especially during
take off? This seems to have been part of Lippisch's thinking in
the X-112, which has a propellor located so most of its outwash
is above the wing. Yet the X-113 and X-114 which should be
advances have the motor rearward and higher, so little wingtop
entrainment and Coanda/Bernoulli lift can occur. Noticeably the
X-112 seems to spend a lot more time at >1/5 wingspan elevations
where the later models seem to fly lower, tighter to the water --
that is, reliant more on chord-based ground effect lift. The
X-112 flies away from the water comfortably.
- I just had an idea. Suppose you could vastly increase
the chord of the wing to provide a larger blown surface and more
lift during take-off, but at the same time vastly decrease the
chord of the wing to spend more of the propellor energy on
propulsion instead of lift. Overlapping "feathers" (slidable
overlapping wing surfaces) on a moving understructure, to shorten
and lengthen the chord. The biplane mentioned herein is an
alternative. Lippisch's pontoon wings added to a stagnation
ground effect reverse delta wing is also a way to deliver both
effects in one vehicle.
- The blown wing top concept is supported by
Channelwing, which increases wing-top air velocity by
creating a venturi effect within a half-channel over the wing,
and demonstrates vertical take off and 8-13 lbs lift per
Where is this going?
This is a big space. Here are some ideas.
Thank you for your interest and patience with this inventory of
incompletely integrated concepts. I invite you to share with me your
thoughts, questions, corrections and friendly suggestions. And
consider, if you are interested in this area, how might we help each
other advance the state of this art?
- Bi-Modal? Consider a vehicle that handles
equally with air propulsion, lift, and control surfaces and
with water propulsion, lift, and control surfaces. Maybe
two engines, two propellers, one for each medium. Maybe air
wings AND hydrofoils, both. Maybe an air rudder like the
giant Jorg tail
AND a water rudder and front-corner sponsons. Effects in
each medium should be coordinated, but can help each other
during transitions, for example, the vehicle will be lifted
by flotation then hydroplaning then aeroplaning with a
weighted mixture of effects adding to the total results for
lift, and similarly for propulsion and control.
- Compromise designs can lose on both fronts, but I would
like to see this for myself rather than give up on the basis
of mere principle. Evidently a specific wave height and
separation distribution should be the basis of design, and
should be practically enforced as hard limits on actual
operation, particularly as to take-off.
- Sponsons I do suggest front right and left corner
sponsons, aerodynamically neutral while providing hydrodynamic
lift and stabilization with smooth rather than abrupt impact
upon entry from free flight; they should be
overall quite small and thin, perhaps knife-like vertical or
which impact the water during flight creating only minimal
drag, up to a certain wave or penetration height. Their
water displacement volume should remain small up to fairly
deep penetrations as through wave tops or to stabilize
flight when rolling onto one side or the other, and get
larger in displacement only when the sponson is operating as
an actual float.
- Some mechanisms for modifying wing/body parameters:
- One is a "shoulder blade", a mechanism for raising the
wing from low-wing (in WIG flight) to high-wing (in
flotation). I don't like the wing stuck in the water
during taxiing or docking, it should be high. Some
rotating truss design should be able to lower the wing
once airborn (to reduce wave battering on the body at a
given wing elevation above the surface).
- Another is a parking mechanism, rotating a single-span
wing back, or folding two spans on each side against
each other like a bird. The bird model is attractive
if probably unrealistic, but should be understood:
after landing they fuss around and fold the wings under
up and back until finally at a low-energy rest
position. Upon takeoff, they sweep backward generating
a backward-rolling horizontal-axis vortex, followed by
a forward sweep making use of the added lift produced
by flying over a vortex that is rolling under your
wing. This is Dr. Wu's point.
- Many such adjustments such as folding multi-span wings
could require quite manipulable wing shapes, and in
turn the feather concept has its value, as feathers
overlappingly form a conjoint surface of manipulable
thickness and shape, each very light and controlled at
the attachment end by shrugs and stretches in
coordination with the whole group to form the wingform
of current utility, depending on landing, lifting off,
soaring, etc. Of course bats fly happily without
feathers, depending on a skin stretch factor that is an
alternative for us also in this design space.
- Don't Blow It Up I don't much like the jets blowing
air under the wings for initial liftoff. Alexeev spent a
lot of time on this one.
- Doesn't fast air above slow air below represent the basic
idea of lift on a curved-top airfoil? Bernoulli, anyone?
Then why blow fast air underneath? It can only work in
stagnation, when the rear edge of the wing is approximately
underwater, then you're blowing up a very leaky balloon with
a jet engine, sure it'll blow up a bit, but it'll spray
water everywhere like crazy and it'll barely work. Mostly
blowing between wing and water amounts to sucking the wing
down to be close to the water. Delifting, not lifting.
Lifting only at the barest beginning, getting the wings just
out of the water. Maybe that's an argument for super low
wings, though, as you see Alexeev's planes were pretty flat
bottomed. But I am reminded of the old two-balloons
experiment: blow between them (fast air) and the balloons
bang together, bang bang bang bang, a little of the pressing
apart happens, but mostly a lot of pulling them together
happens. And then it's a repetitive banging thing. Which
we don't want, neither the pulling two surfaces together
with fast air rushing between them, nor the repetitive
banging. So for me for now, let's scratch the PAR (power
assisted ram) idea, okay? You wiser people, please persuade
me otherwise. Meanwhile, let's forget it.
- PAR is also unnecessary for the purpose of getting the wings
out of the water if the design starts with wings already out
of the water. Then normal propulsion can be used, and any
hydrodynamic design will allow smooth acceleration up to the
point of liftoff. For example, have a high wing on a shoulder
blade, have a water prop pushing at low speeds and jointly
with air propulsion during liftoff acceleration, then let it
spin out and crank up out of the way after liftoff.
- You might even want to force the beast to stay in water
contact until past the point of minimum liftoff speed, in
order to make use of the water propulsion up to a higher
speed, then let it pop up out of the water, letting the
water propulsion system shut down when there is enough power
in the air propulsion system to keep it flying. What would
that take, eh?
- Edderitz Consider as a goal for design a WIG boat with
the abilities of the 2015 Edderitz boat, which keeps stably
close to the surface. The Edderitz boat does a great job
balancing the two forces of air pushing it down to the surface
and water keeping it above the surface. A similar balance,
even more difficult to achieve, would be between the non-lift
at a higher elevation due to being out of ground effect, and
the lift at a lower elevation due to ground effect. For each
velocity there is some optimum, and if stability can be
achieved, more than stability but a set of parameters where
given the velocity then the elevation is specified and there
are strong forces pushing both up and down to keep the vehicle
at that elevation. That's the goal. The Edderitz boat at 1/8
to 1/2 wingspan elevation. How can we achieve that? Fly by
wire? "Pre-tensioned" hydraulic actuators following a balance
point? What are the forces, can we quantify them and design to
float at the right points? That's what I'd like to see.